Steam circuits of this type are known from steam power stations and combined gas and steam power stations, where the thermal energy from steam is converted into kinetic energy in a steam turbine. The steam required to drive the steam turbine is generated in a steam generator from previously purified and desalinated water and overheated in an overheater. The steam is fed from the overheater to the steam turbine, where it releases part of its previously collected thermal energy to the turbine in the form of kinetic energy. A generator is connected to the turbine, which generator transforms the movement of the turbine into electric energy. After flowing through the steam turbine, the decompressed and cooled steam is directed into a condenser, where it cools further by emitting heat and collects in liquid form as water in the so-called hot well. From there the water is pumped via appropriate pumps into a feed water tank and held in reserve there. Finally the condensate is returned to the steam generator via a feed pump. The steam generator itself can be heated using conventional fuels, such as, for example, oil, gas or coal, but can also be heated using nuclear power.
During the operation of the steam circuit, impurities enter into the water used in the circuit, and with time these impurities can result in damage to the steam circuit components. Accordingly, it is necessary to ensure that the chemical, the chemical composition of the circuit medium (water, steam) remains within certain limits. In the case of boilers with cylindrical boiler shells (natural or forced circulation), this is achieved, for example, by water from the drum being blown down constantly or at intervals. In addition, during the starting up and shutting down procedures, water accumulates at the overheater heating surfaces. This water is removed as waste water and must be replaced by treated water (demineralized water). For economical reasons, it is desirable to reduce the proportion of waste water produced and to increase the proportion of reused process waste water. However this is offset by the very high costs involved in the building of the power station, so that with respect to the economic efficiency of the power station as a whole, with the previously known technical options minimizing the waste water arising was not as a rule a good idea. Therefore, in most cases, the steam circuit process waste water produced is just collected and subsequently all thrown away, thus ultimately routed into a waste water system. In most cases, the waste water must undergo a predetermined treatment in accordance with statutory regulations.
In the future, due to a foreseeable further tightening of the terms of environmental protection one can assume that a reduction in the amount of waste water will be enforced by law or that the output of waste water, including conditioning will be made so expensive that a reduction of the amount of waste water will make good economic sense.
In a steam circuit the waste water produced is generally divided into two groups. Draining in the steam area of the steam circuit, such as, for example, draining of the overheater, delivers “clean” waste water, i.e., the chemical composition of the waste water allows it to be reused straight away in the steam circuit. Draining in the water area of the steam circuit, such as, for example, the emergency blow down on the cylindrical boiler shell, produces, in contrast “contaminated” waste water, which means that the chemical composition of the waste water does not permit it to be reused straight away in the steam circuit. The purity of the waste water from the draining in the steam area is based on the fact that during the separation in the steam generator in water and steam phase any impurities in the water phase remain and the steam that leaves the steam generator is clean.
If one is able to collect the clean waste water separately, so that it becomes possible to feed it back into the steam circuit again, then in addition to a reduction of up to 60% in the amount of waste water produced and the expenses related to that, one also saves the corresponding expenses related to the generation and subsequent conditioning of demineralized water that had to replace the discarded water in the circuit.
The greatest proportion of clean waste water occurs at the overheater when starting up and especially when shutting down the power station. This fact makes use of a known concept for minimizing waste water in a steam circuit, wherein the overheater drain lines lead to a separate collector tank. Using a pump the condensate is then pumped from the collector tank into a condensate collector tank and from then on to the condenser of the steam circuit. The known concept is described in more detail below with reference to FIG. 1.